Let's just start with AR-15's.
First off, all AR-15 and other assault style rifles that are semi automatic and fire a specified (to be determined) type of ammunition must be registered in a federal data base. Those who own them now must register their weapons in 30 days or be in violation of the law, which could lead to the confiscation of all your firearms.
Secondly, a mandatory one week waiting period and background check on all of these types of gun purchases.
Third, special permission must be granted for this sale, which is dependent on the background of the purchaser. If they do not meet minimum requirements, they have the option of a gun safety course.
Fourth, all of these rifles can only be sold by someone with an FFL, you cannot purchase these from gun shows or private party sales.
These are just some simple steps that can be taken to prevent these mass slaughters. Look at recent shootings and the guns were purchased legally,, and used within days of purchase. A shooter can still use a pistol or rifle in a mass shooting, but we are trying the limit the damage when this happens.